The present invention relates to transfer paper that is designed to experience a smaller amount of curl upon heat fixing in indirect electrostatic copiers, indirect electrostatic printers and the like (the curl is hereunder referred to as "post fuser curl").
To thermally fuse a toner image in paper on a copier, a printer and the like, it is a common practice to apply heat to one side of the paper. In this case, moisture is removed unevenly from two sideness of the paper and this causes the paper to curl, leading to troubles such as a paper jam and incomplete or inefficient accommodation of paper in the document catching tray or the sorter. If paper absorbs moisture, the paper curls in a particularly great amount on the side opposite the side where copy was made and this increases the chance of the occurrence of troubles. Thus, post fuser curl is an important characteristic that greatly affects the runnability of paper on copiers, printers, etc.
Recent models of medium- and high-speed copiers and printers come in smaller size and yet they feature versatile capabilities such as automatic two-sided copying, multiple color copying and automatic bookbinding. As a result, the complexity of their mechanism has increased and it is required to further reduce the post fuser curl in order to accomplish consistent operations and paper accommodation in the sorter. Take, for example, a machine model that has such paper transport paths that the relationship between the direction in which the copy paper is set in a paper tray and the side of paper where fuser fixing is to be performed varies depending upon which tray is used; with this model, differences will occur on the side of paper where fusing is done and the shape and size of the post fuser curl will vary with the side of paper where copy was made, which can potentially lead to a paper jam or incomplete paper accommodation. In addition, as the frequency of two-sided copying increases, a machine that can perform satisfactorily in one-sided copying operations may occasionally fail due to such troubles as a paper jam and incomplete paper accommodation.
As a result of these changes in the circumstances under which transfer paper is used, the demand for improving quality of paper has increased year by year and there exists particularly strong need for electrophotographic paper that has a satisfactory curl characteristic regarding moisture absorption and which experiences only a small difference in curl between two sideness.
Various methods have heretofore been proposed with a view to solving the problems that occur in electrophotographic paper on account of post fuser curl. For example, Japanese Patent Examined Publication Nos. Sho. 48-96801, 51-102107 and 54-96107 proposed the incorporation of inorganic or synthetic fibers for the purpose of improving the resistance to post fuser curl. However, when inorganic fibers were incorporated, the strength of paper stock drops so markedly that there occurs problems in terms of papermaking operations and the quality of the paper produced. When synthetic fibers were incorporated, the heat resistance of paper stock decrease, causing other problems such as deformation and shrinkage during fuser fixed. Furthermore, both methods had the disadvantage of a significant increase in cost since they used fibers that are more expensive than wood pulp.
Japanese Laid-Open Patent Application Nos. Hei. 3-287894, 3-287895 and 4-18188 disclosed methods that were intended to improve the anti-curl property of mechanical pulp containing paper by controlling its equilibrium moisture and other characteristics. Japanese Laid-Open Patent Application No. Sho. 57-204057 disclosed a method for improving the anti-curl property by increasing the content of fillers but this approach had problems such as lower stiffness and the formation of paper dust in an increased amount. According to the proposals made in Japanese Laid-Open Patent Application Nos. Hei. 3-236062, 3-243953 and 4-5662, it was attempted to solve the problem of curl by controlling the following three parameters respectively: the difference in shrinkage between two sideness of paper; the residual curvature of paper, and the length of fibers in pulp. However, none of these patents disclosed means that will be effective in eliminating the problem of not only the curl upon moisture absorption but also the difference in curl between two sideness of paper.